A magnetic random access memory (MRAM) is a type of a resistance change memories. As techniques for writing data to an MRAM, there have been known magnetic field writing and spin-transfer torque writing are known. Among these techniques, the spin-transfer torque writing has advantages in higher integration, lower power consumption, and higher performance because of the property of a spin-transfer torque device that a spin injection current is smaller in an amount for magnetization reversal as magnetic bodies become smaller in size.
A spin-transfer torque MTJ (Magnetic Tunnel Junction) element has a stacked structure in which a nonmagnetic barrier layer (an insulating thin film) is sandwiched between two ferromagnetic layers, and stores therein data by a change in a magnetic resistance caused by spin-polarized tunneling. The MTJ element can be switched into a low resistance state or a high resistance state depending on the magnetization orientations of the two ferromagnetic layers. The MTJ element is in a low resistance state when the magnetization orientations of the two ferromagnetic layers are in a parallel state (P state), and in a high resistance state when the magnetization orientations thereof are in an anti-parallel state (AP state).
As for the MRAM of this type, it has been desired to realize mass storage, improved performance, and cost reduction as a result of downscaling. Along with the progress of downscaling, the aspect ratio of contact holes increases, and seams or voids tend to occur in contact plugs made of metal more frequently. When an MTJ element is formed on a contact plug having seams or voids, stepped parts resulting from the seams or voids are sometimes formed in the MTJ element. These stepped parts in the MTJ element cause degradation in characteristics of the MTJ elements.